This invention pertains generally to the field of light-emitting diodes (LEDs). More particularly, this invention relates to polymer LEDs which offer high brightness, high efficiency and extended operating life, and which comprise (a) a transparent hole-injecting anode layer; (b) an emissive layer comprising an electroluminescent polymer; and, (c) an electron-injecting cathode layer; wherein said cathode layer comprises an ultra-thin layer of alkaline earth metal (such as calcium, strontium and barium) having a thickness of about 100 xc3x85 or less, typically from about 15 to about 100 xc3x85.
Throughout this application, various publications, patents, and published patent applications are referred to by an identifying citation; full citations for these documents may be found at the end of the specification immediately preceding the claims. The disclosures of the publications, patents, and published patent specifications referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
Diodes and particularly light-emitting diodes (LEDs) fabricated with conjugated organic polymer layers have attracted attention due to their potential for use in display technology. See, for example, Burroughs et al, 1990 and Braun et al., 1991. Among the promising materials for use as active layers in polymer LEDs are poly(phenylene vinylene), PPV, and soluble derivatives of PPV such as poly(2-methyoxy-5-(2xe2x80x2-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, a semiconducting polymer with an energy gap Eg of xcx9c2.1 eV. This material is described in more detail in Wudl et al., 1993. Another material described as useful in active layers of polymer LEDs is poly(2,5-bis(cholestanoxy)-1,4-phenylene vinylene), BCHA-PPV, a semiconducting polymer with an energy gap Eg of xcx9c2.2 eV. This material is described in more detail in Zhang et al., 1993. Other suitable polymers include, for example, the poly(3-alkylthiophenes) (see, for example, Braun et al, 1992) and related derivatives (see, for example, Berggren et al., 1995); poly(p-phenylene) (see, for example, Grem et al., 1992), and its soluble derivatives (see, for example, Yang et al., 1993); and polyquinoline (see, for example, Parker et al., 1994a). Blends of conjugated semiconducting polymers in non-conjugated host polymers are also useful as the active layers in polymer LEDs (see, for example, Zhang et al., 1994). Also useful are blends comprising two or more conjugated polymers (see, for example, Yu, 1996). Generally, materials for use as active layers in polymer LEDs include semiconducting conjugated polymers, more specifically semiconducting conjugated polymers which exhibit photoluminescence, and still more specifically semiconducting conjugated polymers which exhibit photoluminescence and which are soluble and processible from solution into uniform thin films.
In the field of organic polymer-based LEDs it has been taught in the art to employ a relatively high work function metal as the anode, this high work function anode serving to inject holes into the otherwise filled xcfx80-band of the semiconducting, luminescent polymer. Relatively low work function metals are preferred as the cathode material, this low work function cathode serving to inject electrons into the otherwise empty xcfx80*-band of the semiconducting, luminescent polymer. The holes injected at the anode and the electrons injected at the cathode recombine radiatively within the active layer and light is emitted. The common criteria for suitable electrodes are described in detail by Parker et al., 1994b.
Suitable relatively high work function metals for use as anode materials are transparent conducting thin films of indium/tin-oxide (see, for example, Burroughs et al., 1990; Braun et al., 1991). Alternatively, thin films of polyaniline in the conducting emeraldine salt form can be used (see, for example, Gustafsson et al., 1992; Yang et al., 1994; Yang, 1995; and Yang et al., 1995). Thin films of indium/tin-oxide and thin films of polyaniline in the conducting emeraldine salt form are preferred because, as transparent electrodes, both enable the emitted light from the LED to radiate from the device in useful levels.
Suitable relatively low work function metals for use as cathode materials include calcium, magnesium, and lithium. The thickness of the electron injection cathode layer has typically ranged from 200 to 5000 xc3x85 (see, for example, Vanslyke, 1992; Friend et al., 1993; Nakano et al., 1994; and Kido et al., 1995). A lower limit of 200 to 500 xc3x85 is required in order to form a continuous film (full coverage) for the cathode layer (see, for example, Holmes et al., 1996; Scott et al., 1996; and Parker et al., 1994). In addition to good coverage, thicker cathode layers were believed to provide self-encapsulation to keep oxygen and water vapor away from the active parts of the device.
Alloying of reactive cathode metals with more stable metals, such as aluminum or silver, has been used in attempts to improve the cathode""s environmental stability; however, the resulting cathodes remain unstable to reaction with oxygen and/or water vapor (see, for example, Vanslyke, 1991; Vanslyke et al., 1991; and Heeger et al., 1995).
Among the alkaline earth metals, calcium has been widely used in polymer LEDs. Calcium is known to function as an excellent electron-injecting contact. Although strontium and barium have work functions similar to that of calcium, there are no reports in the scientific literature concerning the use of either strontium or barium as cathode materials for polymer LEDs. This is probably due to the higher chemical reactivity of strontium and barium in comparison with calcium. Even calcium is highly reactive, for example, with oxygen and water vapor at room temperature and even more vigorously at elevated temperatures. Consequently, either relatively thick films ( greater than 1000 xc3x85) have been used to provide some degree of self-encapsulation or alloys with stable metals, such as aluminum, have been used as cathodes for LEDs, but with only limited success.
Despite in the improvements in the construction of polymer LEDs, a persistent problem has been rapid decay of the device efficiency (and light output) during stress, especially at elevated temperature. Thus, there is a need for low work function cathodes for use as electron-injecting contacts in polymer LEDs, which have improved stability to reaction with oxygen and water vapor especially at elevated temperature.
The inventor has discovered the surprising and unexpected result that a cathode comprising an ultra-thin layer of an alkaline earth metal offers significant improvements in stability (e.g., extended stress lifetime) as compared to conventional cathodes fabricated from the same metals (and other low work function metals) but with films of thickness greater than 200 xc3x85 (typically in the range 200 xc3x85 to 5000 xc3x85), the latter being known in the art.
Thus, the present invention pertains generally to cathodes comprising an ultra-thin layer (typically about 15 xc3x85 to about 100 xc3x85) of an alkaline earth metal (e.g., calcium, strontium and barium), which functions as a stable contact for efficiently injecting electrons into an emissive layer comprising an electroluminescent polymer, thus resulting in LEDs with high brightness, high efficiency and extended operating life.
The present invention pertains to polymer light-emitting diodes (LEDs) which have extended operating life, and more particularly, LEDs in which the decay of efficiency and light output is substantially postponed.
Thus, one aspect of the present invention pertains to a polymer light-emitting diode (LED) comprising:
(a) a transparent hole-injecting anode layer;
(b) an emissive layer comprising an electroluminescent polymer; and
(c) an electron-injecting cathode layer;
wherein said cathode layer comprises an ultra-thin layer of alkaline earth metal having a thickness of from about 15 to about 100 xc3x85.
In one embodiment, the alkaline earth metal is selected from the group consisting of calcium, strontium, and barium.
In another embodiment, the ultra-thin layer of alkaline earth metal has a thickness of from about 30 to about 60 xc3x85.
In another embodiment, the cathode layer further comprises a capping layer. In one embodiment, the capping layer comprises aluminum, silver, or copper.
In another embodiment, the electroluminescent polymer is a conjugated polymer. In one embodiment, the electroluminescent polymer is selected from the group consisting of: poly(p-phenylene vinylene)s, poly(arylene vinylene)s, poly(p-phenylene)s, poly(arylene)s, and polyquinolines. In one embodiment, the electroluminescent polymer is poly(2-(3,7-dimethyloctyloxy)-5-methoxy-1,4-phenylene vinylene).
In another embodiment, the anode layer comprises a material selected from the group consisting of: metal, metal oxide, graphite, doped inorganic semiconductor, doped conjugated polymer. In one embodiment, the anode layer comprises a material selected from the group consisting of: aluminum, silver, platinum, gold, palladium, tungsten, indium, copper, iron, nickel, zinc, lead, tin oxide, indium/tin oxide, graphite, doped silicon, doped germanium, doped gallium arsenide, doped polyaniline, doped polypyrrole, and doped polythiophene.
In another embodiment, the LED is encapsulated.